1. Field of the Invention
The present invention relates generally to orientation and/or position referencing systems, more particularly, to systems and methods for providing orientation and/or position referencing systems with scanning polarized RF reference sources.
2. Prior Art
For guidance and/or steering purposes, all manned and unmanned mobile platforms, such as land vehicles, powered or non-powered airborne platforms, surface marine platforms, or various space vehicles, require onboard information as to their absolute position and/or orientation or their position and/or orientation relative to another object such as a reference platform or a target object. Here, by absolute position and/or orientation are meant to be relative to a reference coordinate system such as earth or one established by other means such as the present polarized RF reference sources.
This position and orientation information is particularly important for unmanned and guided platforms such as mobile robots, Unmanned Aerial Vehicles (UAV), Unmanned Ground Vehicles (UGV), and the like. This is also the case in future smart and guided projectiles, including gun-fired munitions, mortars, missiles and gravity dropped weapons. Such platforms will also require the aforementioned absolute and/or relative position and orientation information onboard the platform for closing the feedback guidance and control loop to guide the platform to the desired target or track a specified trajectory or the like.
In certain cases, the onboard position and certain orientation (usually not full orientation) information (absolute or relative to the target, a reference station, another mobile platform, etc.) can be provided by an outside source, for example, by GPS for position or by a radar reading or optical signal that is reflected off some target or received by the mobile platform. In other cases, it is either required or is highly desirable to have autonomous sensors onboard the mobile platform, including gun-fired projectiles, mortars and missiles, UAVs and UGVs, to directly measure the position and orientation of the object with respect to a fixed object (for example a ground station) or a moving object (for example a moving target).
It is noted that even though in this disclosure all references are made to moving platforms, it will be appreciated by those of ordinary skill in the art that the provided description also includes the measurement of the position and orientation of one object relative to another object, one or both of which may be fixed to a third object such as the ground.
Currently available sensors for remote measurement of the angular orientation of an object relative to the earth or another object (target or weapon platform) can be divided into the following five major classes.
The first class of sensors measure changes in the angular orientation using inertial devices such as accelerometers and gyros of various types. Inertial based angular orientation sensors, however, generally suffer from noise and drift error accumulation and settling time problems, particularly for gun-fired munitions and mortars. In such sensors, the drift and the measurement errors are accumulated over time since the measured acceleration has to be integrated to determine the position and angular orientation information. As a result, the error in the position and angular orientation measurements increases over time. In addition, the initial position (angular orientation) and velocity (angular velocity) of the object must be known accurately. Another shortcoming of inertia based position and angular orientation sensors is that the position and angular orientation of one object relative to another object cannot be measured directly, i.e., the position and angular orientation of each object relative to the inertia frame (earth in most cases) have to be measured separately and used to determine their relative position and angular orientation. As a result, errors in both measurements are included in the relative position and angular orientation measurements, thereby increasing it even further. In addition, electrical energy has to be spent during the entire time to continuously make such sensory measurements.
In the particular case of gun-fired munitions and mortars, two other major problems are encountered with inertia-based sensors. Firstly, they have to be made to withstand firing accelerations that in certain cases could be in excess of 100,000 Gs. However, to achieve the required guidance and control accuracy over relatively long distances and related times, the absolute angular orientation of the projectile has to be known during the entire time of the flight within very small angles corresponding to sub-fractions of one G. As a result, the accelerometers and gyros are prone to a settling time problem, particularly with the aforementioned initial high G loading. In certain cases, when GPS signal is available, it can be used to initialize the position information in accelerometers. The GPS, however, cannot be used to initialize gyros (except for orientation in the horizontal plane). Obviously, the development of inertia based accelerometers and gyros that could withstand the aforementioned high G levels and require near zero settling time is an extremely difficult task.
The second class of angular orientation sensors operates using optical methods. Such sensory systems can directly measure angular position of one object relative to another. As such, the optical sources used for such measurements can be used to set up a reference coordinate system relative to which the corresponding sensors on an object can measure the object position and/or orientation. However, optical based angular position sensory systems suffer from several disadvantages, including operation only in the line of sight between the two objects; accurate measurement of relative angular orientation only if the objects are relatively close to each other; limited range of angular orientation measurement; relatively high power requirement for operation; requirement of relatively clean environment to operate; and in military applications the possibility of exposing the site to the enemy and jamming. Optical gyros do not have most of the above shortcomings but are relatively large, require a considerable amount of power, expensive, and are difficult to harden for high G firing accelerations. Optical methods such as tracking of projectiles with surface mounted reflectors and the like have also been developed, which are extremely cumbersome to use even during verification testing, suffer from all the aforementioned shortcomings, and are impractical for fielded munitions. In addition, the information about the object orientation can usually be determined only at the ground station and has to be transmitted to the moving object for guidance and control purposes. As a result, optical angular position sensors are generally not suitable for munitions and other similar applications.
The third class of angular orientation sensors is magnetometers that can be used to measure orientation relative to the magnetic field of the earth. The main problem with magnetometers is that they cannot measure orientation of the object about the magnetic field of the earth. Other important issues are low sensitivity; requirement of an accurate map of the magnetic field in the area of operation; and sensitivity to the presence of vehicles and the like in the area, the configuration of which usually varies in time, particularly in an active war theatre.
The fourth class of angular orientation measurement systems are based on the use of radio frequency (RF) antennas printed or placed on the surface of an object to reflect RF energy emanating from a ground-based radar system. The reference coordinate system in which the position and/or orientation of the object is measured is thereby set up by the said (generally ground-based) radar system. The reflected energy is then used to track the object on the way to its destination. With two moving objects, the radar measures the time difference between the return signals from each of the objects and thereby determines angular information in terms of the angle that the relative velocity vector makes with respect to a coordinate system fixed to one of the objects. With such systems, measurement of full spatial orientation of an object (relative to the fixed radar or a second object) is very difficult. In addition, the information about the object orientation is determined at the radar station and has to be transmitted back to the moving object(s) if it is to be used for course correction. It is also very difficult and costly to develop systems that could track multiple projectiles. It is noted that numerous variations of the above method and devices have been devised with all suffering from similar shortcomings.
In addition to the above angular orientation measurement sensors, GPS signals can generally be only used to measure the direction of motion of the object in the horizontal plane. The reference coordinate system is thereby fixed to the earth. The GPS, however, has a number of significant shortcomings, particularly for munitions applications in general and gun fired munitions and mortars in particular. These include the fact that GPS signals may not be available along the full path of the flight; and the measurements cannot be made fast enough to make them suitable for guidance and control purposes in gun fired munitions and mortars. In addition, GPS signals are generally weak and prone to jamming.
The fifth class of angular orientation sensors is based on utilizing polarized Radio Frequency (RF) reference sources and mechanical cavities as described in U.S. Pat. Nos. 6,724,341 and 7,193,556 and U.S. Pat. No. 7,425,918, all of which are incorporated herein by reference, and hereinafter are referred to as “polarized RF angular orientation sensors”. These angular orientation sensors use highly directional mechanical cavities that are very sensitive to the orientation of the sensor relative to the reference source due to the cross-polarization and due to the geometry of the cavity. The reference source may be fixed on the ground or may be another mobile platform (object). Being based on RF carrier signals, the sensors provide a longer range of operation. The sensors can also work in and out of line of sight. In addition, the sensors make angular orientation measurements directly and would therefore not accumulate measurement error. The sensor cavities receive and record the electromagnetic energy emitted by one or more polarized RF sources. The angular position of a cavity sensor relative to the reference source is indicated by the energy level that it receives. A system equipped with multiple such cavity sensors can then be used to form a full spatial orientation sensor. In addition, by providing more than one reference source, full spatial position of munitions can also be measured onboard the munitions.
The aforementioned polarized RF based angular orientation sensors provide highly precise angular orientation measurements. The sensors, when embedded in a mobile platform such as in a projectile, can measure full angular orientation of the projectile (mobile platform) relative to the reference coordinate system established by the polarized RF reference sources, which may be fixed to a ground station or to another moving object such as a UAV or UGV or another projectile (mobile platform) or the like. These angular orientation sensors are autonomous, i.e., they do not acquire sensory information through communication with a ground, airborne or the like sources. The sensors are relatively small and can be readily embedded into the structure of most mobile platforms including munitions without affecting their structural integrity. As a result, such sensors are inherently shock, vibration and high G acceleration hardened. A considerable volume is thereby saved for use for other gear and added payload. In addition, the sensors become capable of withstanding environmental conditions such as moisture, water, heat and the like, even the harsh environment experienced by munitions during firing. In addition, the sensors require a minimal amount of onboard power to operate, particularly since they only need to be powered when the sensory information is required.
The latter two classes of RF based full angular orientation and full position sensors promise to provide low cost, small volume and lightweight, low power, precision and autonomous onboard sensors for guidance and control of all mobile platforms, including future smart and precision guided munitions, as alternatives to inertia-based, optical, GPS and other similar currently available sensors.
The latter two classes of RF based full angular orientation sensors are dependent on the magnitude of the received signal at the sensors from the reference sources to determine the orientation of the sensor relative to the polarized RF reference source, i.e., relative to the reference coordinate system established by the polarized RF reference sources. This is the case, for example, for the aforementioned angular orientation sensors which are based on utilizing polarized Radio Frequency (RF) reference sources and mechanical cavities as described in U.S. Pat. Nos. 6,724,341 and 7,193,556 and U.S. Pat. No. 7,425,918.
Briefly, referring now to FIGS. 1 and 2, there is shown a representation of a polarized RF cavity sensor 100 and its operation with respect to a polarized radio frequency (RF) reference (illuminating) source 101. An electromagnetic wave consists of orthogonal electric (E) and magnetic (H) fields. The electric field E and the magnetic field H of the illuminating beam are mutually orthogonal to the direction of propagation of the illumination beam. When polarized, the planes of E and H fields are fixed and stay unchanged in the direction of propagation. Thus, the illuminating source establishes a (reference) coordinate system with known and fixed orientation. The polarized RF cavity sensor 100 reacts in a predictable manner to a polarized illumination beam. When three or more polarized RF cavity sources are distributed over the body of an object, and when the object is positioned at a known distance from the illuminating source, the amplitudes of the signals received by the polarized RF cavity sensors 100 can be used to determine the orientation of the object relative to the reference (illuminating) source 101, i.e., in the aforementioned reference coordinate system of the reference source 101. The requirement for the proper distribution of the polarized RF cavity sensors 100 over the body of the object is that at least three of the polarized RF cavity sensors be neither parallel nor co-planar.
It is therefore observed that the aforementioned classes of polarized RF based full angular orientation sensors are dependent on the magnitude of the received signal at the sensors from the polarized RF reference sources to determine the orientation of the sensors relative to the reference sources, i.e., relative to the reference coordinate system established by the polarized RF reference sources. The use of the signal magnitude, however, has several major shortcomings that limits the utility of such sensors as well as degrades their angular orientation measurement precision. The following are the major shortcomings of the aforementioned use of signal magnitude information in these sensors for measuring angular orientation relative to the polarized RF reference sources, i.e., relative to the reference coordinate system established by the polarized RF reference sources:                1. To relate the magnitude of the received signal to angular orientation, the distance from the reference source to the angular orientation sensors must be known. This in general means that other means have to be also provided to measure or indicate the position of the orientation sensor relative to the reference source. The position information may, for example, be provided by GPS or as mentioned in the aforementioned U.S. Pat. Nos. 6,724,341 and 7,193,556 by using three reference sources and triangulation techniques.        2. In practice, the signal received at the angular orientation sensor would be very noisy and face losses due to the environmental conditions and reflections, and is also prone to measurement errors at the sensors.        3. The magnitude of the signal received at the sensors and its relationship to the angular orientation of the sensors (object to which the sensors are attached) could be significantly different when the object is not in the line-of-sight of the reference source. Therefore when the object is not in the line-of-sight, the use of the received signal magnitude information could in general lead to significant degradation of the accuracy of the angular orientation measurements.        